An ultrasonic proximity transducer repeatedly transmits ultrasonic pulses of equal width and subsequently outputs analog signals that are received, some of which contain an associated usable echo signal. The received analog signals are evaluated in a control and evaluation unit which takes into consideration deviations in the usable echo signals that are due to interference signals.
Interference suppression methods for ultrasonic proximity transducers are known, for example, from DE No. 34 28 773. In that reference, error pulses are eliminated using pulse width modulation wherein it is assumed that the transmitter always produces ultrasonic pulses of equal width. Error pulses are recognized since they are narrower or wider than the transmitted ultrasonic pulses. The only pulses considered for usable echo signals are those which exceed a constant error threshold. This serves to suppress interference described in the book Halbleiter Schaltungtechnik (Semiconductor Circuitry) by U. Tietze, Ch. Schenk on page 411.